Electrochemical cells comprising laminar flow induced dynamic conducting interfaces, electronic devices comprising such cells, and methods employing same

ABSTRACT

An electrochemical cell is described that includes (a) a first electrode; (b) a second electrode; and (c) a channel contiguous with at least a portion of the first and the second electrodes. When a first liquid is contacted with the first electrode, a second liquid is contacted with the second electrode, and the first and the second liquids flow through the channel, a parallel laminar flow is established between the first and the second liquids. Electronic devices containing such electrochemical cells and methods for their use are also described.

BACKGROUND

This invention relates to the field of induced dynamic conductinginterfaces. More particularly, this invention relates to laminar flowinduced dynamic conducting interfaces for use in micro-fluidicbatteries, fuel cells, and photoelectric cells.

A key component in many electrochemical cells is a semi-permeablemembrane or salt bridge. One of the primary functions of thesecomponents is to physically isolate solutions or solids having differentchemical potentials. For example, fuel cells generally contain asemi-permeable membrane (e.g., a polymer electrolyte membrane or PEM)that physically isolates the anode and cathode regions while allowingions (e.g., hydrogen ions) to pass through the membrane. Unlike theions, however, electrons generated at the anode cannot pass through thismembrane, but instead travel around it by means of an external circuit.Typically, semi-permeable membranes are polymeric in nature and havefinite life cycles due to their inherent chemical and thermalinstabilities. Moreover, such membranes typically exhibit relativelypoor mechanical properties at high temperatures and pressures, whichseriously limits their range of use.

Fuel cell technology shows great promise as an alternative energy sourcefor numerous applications. Several types of fuel cells have beenconstructed, including: polymer electrolyte membrane fuel cells, directmethanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells,molten carbonate fuel cells, and solid oxide fuel cells. For acomparison of several fuel cell technologies, see Los Alamos NationalLaboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power bySharon Thomas and Marcia Zalbowitz, the entire contents of which areincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present application, thedisclosure or definition herein shall be deemed to prevail.

Although all fuel cells operate under similar principles, the physicalcomponents, chemistries, and operating temperatures of the cells varygreatly. For example, operating temperatures can vary from roomtemperature to about 1000° C. In mobile applications (for example,vehicular and/or portable microelectronic power sources), afast-starting, low weight, and low cost fuel cell capable of high powerdensity is required. To date, polymer electrolyte fuel cells (PEFCs)have been the system of choice for such applications because of theirlow operating temperatures (e.g., 60-120° C.), and inherent ability forfast start-ups.

FIG. 1 shows a cross-sectional schematic illustration of a polymerelectrolyte fuel cell 2. PEFC 2 includes a high surface area anode 4that acts as a conductor, an anode catalyst 6 (typically platinumalloy), a high surface area cathode 8 that acts as a conductor, acathode catalyst 10 (typically platinum), and a polymer electrolytemembrane (PEM) 12 that serves as a solid electrolyte for the cell. ThePEM 12 physically separates anode 4 and cathode 8. Fuel in the gasand/or liquid phase (typically hydrogen or an alcohol) is brought overthe anode catalyst 6 where it is oxidized to produce protons andelectrons in the case of hydrogen fuel, and protons, electrons, andcarbon dioxide in the case of an alcohol fuel. The electrons flowthrough an external circuit 16 to the cathode 8 where air, oxygen, or anaqueous oxidant (e.g., peroxide) is being constantly fed. Protonsproduced at the anode 4 selectively diffuse through PEM 12 to cathode 8,where oxygen is reduced in the presence of protons and electrons atcathode catalyst 10 to produce water.

The PEM used in conventional PEFCs is typically composed of aperfluorinated polymer with sulphonic acid pendant groups, such as thematerial sold under the tradename NAFION by DuPont (Fayetteville, N.C.)(see: Fuel Cell Handbook, Fifth Edition by J. Hirschenhofer, D.Stauffer, R. Engleman, and M. Klett, 2000, Department of Energy—FETL,Morgantown, W. Va.; and L. Carrette, K. A. Friedrich, and U. Stimming inFuel Cells, 2001, 1(1), 5). The PEM serves as catalyst support material,proton conductive layer, and physical barrier to limit mixing betweenthe fuel and oxidant streams. Mixing of the two feeds would result indirect electron transfer and loss of efficiency since a mixed potentialand/or thermal energy is generated as opposed to the desired electricalenergy.

Operating the cells at low temperature does not always proveadvantageous. For example, carbon monoxide (CO), which may be present asan impurity in the fuel or as the incomplete oxidation product of analcohol, binds strongly to and “poisons” the platinum catalyst attemperatures below about 150° C. Therefore, CO levels in the fuel streammust be kept low or removed, or the fuel must be completely oxidized tocarbon dioxide at the anode. Strategies have been employed either toremove the impurities (e.g., by an additional purification step) or tocreate CO-tolerant electrodes (e.g., platinum alloys). In view of thedifficulties in safely storing and transporting hydrogen gas, the lowerenergy density per volume of hydrogen gas as compared to liquid-phasefuels, and the technological advances that have occurred in preparingCO-tolerant anodes, liquid fuels have become the phase of choice formobile power sources.

Numerous liquid fuels are available. Notwithstanding, methanol hasemerged as being of particular importance for use in fuel cellapplications. FIG. 2 shows a cross-sectional schematic illustration of adirect methanol fuel cell (DMFC) 18. The electrochemical half reactionsfor a DMFC are as follows: $\frac{\begin{matrix}{\quad {{{{Anode}\text{:}\quad {CH}_{3}{OH}} + {H_{2}O}}\overset{\quad}{\overset{\quad}{\rightarrow}}{{CO}_{2} + {6H^{+}} + {6e^{-}}}}\quad} \\{\quad \left. {{{Cathode}\text{:}\quad {3/2}\quad O_{2}} + {6H^{+}} + {6e^{-}}}\rightarrow{3\quad H_{2}O} \right.\quad}\end{matrix}\quad}{\quad \left. {{{Cell}\quad {Reaction}\text{:}\quad {CH}_{3}{OH}} + {{3/2}\quad O_{2}}}\rightarrow{{CO}_{2} + {2H_{2}O}} \right.}$

As shown in FIG. 2, the cell utilizes methanol fuel directly, and doesnot require a preliminary reformation step. DMFCs are of increasinginterest for producing electrical energy in mobile power (low energy)applications. However, at present, several fundamental limitations haveimpeded the development and commercialization of DMFCs.

One of the major problems associated with DMFCs is that thesemi-permeable membrane used to separate the fuel feed (i.e., methanol)from the oxidant feed (i.e., oxygen) is typically a polymer electrolytemembrane (PEM) of the type developed for use with gaseous hydrogen fuelfeeds. These PEMs, in general, are not fully impermeable to methanol. Asa result, an undesirable occurrence known as “methanol crossover” takesplace, whereby methanol travels from the anode to the cathode throughthe membrane. In addition to being an inherent waste of fuel, methanolcrossover also causes depolarization losses (mixed potential) at thecathode and, in general, leads to decreased cell performance.

Therefore, in order to fully realize the promising potential of DMFCs ascommercially viable portable power sources, the problem of methanolcrossover must be addressed. Moreover, other improvements are alsoneeded including: increased cell efficiency, reduced manufacturingcosts, increased cell lifetime, and reduced cell size/weight. In spiteof massive research efforts, these problems persist and continue toinhibit the commercialization and development of DMFC technology.

A considerable amount of research has already been directed at solvingthe aforementioned problem of methanol crossover. Solutions havetypically centered on attempts to increase the rate of methanolconsumption at the anode, and attempts to decrease the rate of methanoldiffusion to the cathode (see: A. Heinzel, and V. M. Barragan in J.Power Sources, 1999, 84, 70, and references therein). Strategies forincreasing the rate of methanol consumption at the anode have includedincreasing catalyst loading (i.e., providing a larger surface area),increasing catalyst activity (i.e., increasing efficiency), and raisingoperating pressure and/or temperature. Strategies for decreasing therate of methanol diffusion to the cathode have included decreasingmethanol concentrations, fabricating thicker NAFION membranes,synthesizing new proton conducting materials having low permeability tomethanol, lowering cell operating temperature, and fabricating methanoltolerant cathodes. However, to date, there remain pressing needs in DMFCtechnology for significantly lowered fabrication costs, increasedefficiency, extended cell lifetimes, and appreciably reduced cellsizes/weights.

SUMMARY

The scope of the present invention is defined solely by the appendedclaims, and is not affected to any degree by the statements within thissummary.

In a first aspect, the present invention provides an electrochemicalcell that includes (a) a first electrode; (b) a second electrode; and(c) a channel contiguous with at least a portion of the first and thesecond electrodes; such that when a first liquid is contacted with thefirst electrode, a second liquid is contacted with the second electrode,and the first and the second liquids flow through the channel, aparallel laminar flow is established between the first and the secondliquids, and a current density of at least 0.1 mA/cm²is produced.

In a second aspect, the present invention provides a device thatincludes an electrochemical cell as described above.

In a third aspect, the present invention provides a portable electronicdevice that includes an electrochemical cell as described above.

In a fourth aspect, the present invention provides a method ofgenerating an electric current that includes operating anelectrochemical cell as described above.

In a fifth aspect, the present invention provides a method of generatingwater that includes operating an electrochemical cell as describedabove.

In a sixth aspect, the present invention provides a method of generatingelectricity that includes flowing a first liquid and a second liquidthrough a channel in parallel laminar flow, wherein the first liquid isin contact with a first electrode and the second liquid is in contactwith a second electrode, wherein complementary half cell reactions takeplace at the first and the second electrodes, respectively, and whereina current density of at least 0.1 mA/cm² is produced.

In a seventh aspect, the present invention provides a fuel cell thatincludes a first electrode and a second electrode, wherein ions travelfrom the first electrode to the second electrode without traversing amembrane, and wherein a current density of at least 0.1 mA/cm² isproduced.

In an eighth aspect, the present invention provides the improvementcomprising replacing the membrane separating a first and a secondelectrode of a fuel cell with a parallel laminar flow of a first liquidcontaining a fuel in contact with the first electrode, and a secondliquid containing an oxidant in contact with the second electrode, andproviding each of the first liquid and the second liquid with a commonelectrolyte.

In a ninth aspect, the present invention provides an electrochemicalcell that includes (a) a support having a surface; (b) a first electrodeconnected to the surface of the support; (c) a second electrodeconnected to the surface of the support and electrically coupled to thefirst electrode; (d) a spacer connected to the surface of the support,which spacer forms a partial enclosure around at least a portion of thefirst and the second electrodes; and (e) a microchannel contiguous withat least a portion of the first and the second electrodes, themicrochannel being defined by the surface of the support and the spacer.When a first liquid is contacted with the first electrode, and a secondliquid is contacted with the second electrode, a parallel laminar flowis established between the first and the second liquids, and a currentdensity of at least 0.1 mA/cm² is produced.

The presently preferred embodiments described herein may possess one ormore advantages relative to other devices and methods, which can includebut are but not limited to: reduced cost; increased cell lifetime;reduced internal resistance of the cell; reduction or elimination ofmethanol crossover or fouling of the cathode; ability to recycleleft-over methanol that crosses over into the oxidant stream back intothe fuel stream; ability to increase reaction kinetics proportionallywith temperature and/or pressure without compromising the integrity of amembrane; and ability to fabricate a highly efficient, inexpensive, andlightweight cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional schematic illustration of a polymerelectrolyte fuel cell.

FIG. 2 shows a cross-sectional schematic illustration of a directmethanol fuel cell.

FIG. 3 shows a schematic illustration of modes of fluid flow.

FIG. 4 shows a schematic illustration of the relationship between inputstream geometry and mode of fluid flow.

FIG. 5 shows a schematic illustration of the relationship betweenmicrofluidic flow channel geometry and mode of fluid flow.

FIG. 6 shows a schematic illustration of a diffusion-basedmicro-extractor.

FIG. 7 shows a schematic illustration of a direct methanol fuel cellcontaining a laminar flow induced dynamic interface.

FIG. 8 shows a schematic illustration of side-by-side and face-to-facemicrofluidic channel configurations.

FIG. 9 shows a perspective view of a laminar flow fuel cell inaccordance with the present invention.

FIG. 10 shows an exploded perspective view of the fuel cell shown inFIG. 9.

FIG. 11 shows a plot of current vs. voltage for a copper-zinc laminarflow fuel cell.

FIG. 12 shows a plot of current vs. voltage for a platinum-platinumlaminar flow fuel cell.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A revolutionary paradigm in cell design, which solves many of theproblems described above, has been discovered wherein the use of a PEMhas been eliminated entirely. An electrochemical cell in accordance withthe present invention does not require a membrane, and is therefore notconstrained by the limitations inherent in conventional membranes.Instead, a mechanism has been developed by which ions can travel fromone electrode to another without traversing a membrane, and which allowsproton conduction while preventing mixing of the fuel and oxidantstreams. This mechanism, described more fully hereinbelow, involvesestablishing laminar flow induced dynamic conducting interfaces.

Throughout this description and in the appended claims, the phrase“electrochemical cell” is to be understood in the very general sense ofany seat of electromotive force (as defined in Fundamentals of Physics,Extended Third Edition by David Halliday and Robert Resnick, John Wiley& Sons, New York, 1988, 662 ff.). The phrase “electrochemical cell”refers to both galvanic (i.e., voltaic) cells and electrolytic cells,and subsumes the definitions of batteries, fuel cells, photocells(photovoltaic cells), thermopiles, electric generators, electrostaticgenerators, solar cells, and the like. In addition, throughout thisdescription and in the appended claims, the phrase “complementary halfcell reactions” is to be understood in the very general sense ofoxidation and reduction reactions occurring in an electrochemical cell.

Ideally, the structural components of a DMFC will have the followingcharacteristics. Preferably, the membrane should (1) be resistant toharsh oxidizing/reducing environments; (2) possess mechanical toughness;(3) be resistant to high temperatures and pressures (e.g., 0-160° C. and1-10 atm); (4) be impermeable to methanol under all operatingconditions; (5) conduct protons with minimal ohmic resistance and masstransport losses; and (6) be composed of lightweight and inexpensivematerials. Both the anode and cathode, preferably, should (1) exhibithigh catalytic activity; (2) possess a large surface area; (3) requireminimal amounts of precious metals; and (4) be easily to fabricated. Inaddition, the anode should preferably show tolerance to carbon monoxide,and the cathode should preferably show tolerance to methanol if soneeded. The integrated fuel cell assembly itself should preferably (1)have few moving parts; (2) require no external cooling system; (3)require no fuel reformer or purifier; (4) be composed of durable andinexpensive components; (5) be easily fabricated; (6) be easilyintegrated into fuel cell stacks; and (7) provide highly efficientenergy conversion (i.e., at least 50%).

Heretofore, there has been no single fuel cell design that successfullyincorporates all of the aforementioned attributes. However, it has nowbeen discovered that by completely eliminating the PEM from the DMFC,and by redesigning the system to function on the microfluidic scale, oneor more of these attributes can be achieved. In the absence of a PEM, amechanism to allow proton conduction while preventing mixing of the fueland oxidant streams is needed. Such a mechanism, described more fullyhereinbelow, can be established in microfluidic flow channels through aphenomenon known as “parallel laminar flow,” whereby two liquid streamsflow side-by-side in physical contact (thereby enabling protonconduction), without mixing and in the complete absence of a physicalbarrier or membrane. The two liquids can be miscible or immiscible.Obviation of a physical membrane for stream segregation and protontransport from a fuel cell significantly decreases manufacturing costsand increases the efficiency and versatility of the cell.

As shown in FIG. 3, fluid flow can be categorized into two regimes:laminar flow and turbulent flow. In steady or laminar flow (FIG. 3A),the velocity of the fluid at a given point does not change with time(i.e., there are well-defined stream lines). In turbulent flow (FIG.3B), the velocity of the fluid at a given point does change with time.While both laminar and turbulent flow occur in natural systems (e.g., inthe circulatory system), turbulent flow generally predominates on themacroscale. In contrast, laminar flow is generally the norm on themicrofluidic scale.

An indicator of the relative turbulence of a flow stream for a fluidunder flow can be expressed as a dimensionless quantity known as theReynolds number (R_(e)). The Reynolds number is defined as the ratio ofinertial forces to viscous forces, and can be expressed as:

R _(e) =ρL/μ

where L is the characteristic length in meters, ρ is the density of thefluid in grams/cm³, v is the linear velocity in meters/sec, and μ is theviscosity of the fluid in grams/(sec)(cm).

There is a transitional critical value of R_(e) for any given geometryabove which flow is said to be turbulent and below which flow is said tobe laminar. For typical fluidic devices, the transition from laminar toturbulent flow has been empirically determined to occur aroundR_(e)=2,300. Formulae to calculate R_(e) for specific geometries arewell known (see: Micromachined Transducers: Sourcebook by G. T. A.Kovacs, McGraw-Hill, Boston, 1998). In some microchannel geometries,flow is strictly laminar, complicating the mixing of two misciblestreams. However, as shown in FIG. 4, the geometry of the input streamscan greatly affect turbulence and mixing. A T-junction (FIG. 4A) bringstwo miscible streams together in a laminar flow, which is maintainedwithout turbulent mixing. In contrast, introducing the two streams in anarrow-type junction (FIG. 4B) produces turbulent flow and subsequentmixing.

In addition to the influence of input stream geometry on the mode offluid flow, the geometry of the microfluidic channel also has an effect.The mixing efficiencies of various channel shapes have beeninvestigated, as shown in FIG. 5 (see: J. Branebjerg, B. Fabius, and P.Gravesen, “Application of Miniature Analyzers from MicrofluidicComponents to μTAS,” in Proceedings of Micro Total Analysis SystemsConference, Twente, Netherlands, Nov. 21-22, 1994, edited by A. van denBerg and P. Bergveld, pp 141-151). Brothymol Blue (yellowish) wasinjected into one of the input ports and NaOH into the other. Mixingcould be observed by the formation of a dark blue product. Resultsindicated that turbulent flow was caused by sharp corners, whichresulted in full mixing by the time the fluid had traversed about onethird of the zigzag pattern channel (FIG. 5B). However, since the onlymode of mixing possible for the straight channel was diffusion, nomixing was observed at the same flow rate (FIG. 5A).

Geometry is not the only variable that affects the degree of mixing. Theresidence time, or flow rates of solutions can have an impact as well.The average time for a particle to diffuse a given distance depends onthe square of that distance. A diffusion time scale (T_(d)) can beexpressed as

T _(d) =L ² /D

where L is the relevant mixing length in micrometers and D is thediffusion coefficient. The rate of diffusion for a given molecule istypically determined by its size. A table of diffusion coefficients forsome common molecules is shown below in Table 1 (see: J. P. Brody, andP. Yager, “Diffusion-Based Extraction in a Microfabricated Device,”Sensors and Actuators, January, 1997, A58, no. 1, pp. 13-18). As may beseen from this Table, the proton (H⁺) has the highest diffusioncoefficient in water at room temperature.

TABLE 1 Diffusion Coefficient Molecular Weight In Water at Room TempWater Soluble Molecule (AMU) (μm²/sec) H⁺  1 9,000 Na⁺ 23 2,000 O₂ 321,000 Glycine 75 1,000 Hemoglobin 6 × 10⁴ 70 Myosin 4 × 10⁵ 10 TobaccoMosaic Virus 4 × 10⁷ 5

When two fluids with differing concentrations or compositions ofmolecules are forced to flow parallel to one another in a singlechannel, extraction of molecules can be accomplished on the basis ofdiffusion coefficient differences. For example, as shown in FIG. 6, Na⁺can be extracted from blood plasma by controlling channel dimension,flow rate, and the dwell time the two streams are in contact, thusproducing a continuous micro-extractor (see: Brody reference, videsupra).

It has been discovered that parallel laminar flow between two misciblestreams of liquid induces an ultra-thin dynamic conducting(“semi-permeable”) interface (hereinafter “induced dynamic conductinginterface” or “IDCI”), which wholly replaces the PEMs or salt bridges.of conventional devices. The IDCI can maintain concentration gradientsover considerable flow distances and residence times depending on thedissolved species and the dimensions of the flow channel.

An electrochemical cell embodying features of the present inventionincludes (a) a first electrode; (b) a second electrode; and (c) achannel contiguous with at least a portion of the first and the secondelectrodes. When a first liquid is contacted with the first electrode, asecond liquid is contacted with the second electrode, and the first andthe second liquids flow through the channel, a parallel laminar flow isestablished between the first and the second liquids, and a currentdensity of at least 0.1 mA/cm² is produced.

Flow cell designs embodying features of the present invention introducea new paradigm for electrochemical cells in general, and for fuelcells—especially DMFCs—in particular. A fuel cell 20 embodying featuresof the present invention that does not require a PEM nor is subject toseveral of the limitations imposed by conventional PEMs is shown in FIG.7. In this design, both the fuel input 22 (e.g. an aqueous solutioncontaining MeOH and a proton source) and the oxidant input 24 (e.g., asolution containing dissolved oxygen or hydrogen peroxide and a protonsource) are in liquid form. By pumping the two solutions into themicrochannel 26, parallel laminar flow induces a dynamic protonconducting interface 28 that is maintained during fluid flow. If theflow rates of the two fluids are kept constant and the electrodes areproperly deposited on the bottom and/or top surfaces of the channel, theIDCI is established between anode 30 and cathode 32.

A proton gradient is created between the two streams and rapid protondiffusion completes the circuit of the cell as protons are produced atanode 30 and consumed at cathode 32. In this case, the IDCI prevents thetwo solutions from mixing and allows rapid proton conduction bydiffusion to complete the circuit.

Preferably, the liquid containing the fuel and the liquid containing theoxidant each contains a common electrolyte, which is preferably a sourceof protons (e.g., a Brønsted acid). A portion of these externallyprovided protons may be consumed in the half-cell reaction occurring atthe cathode. Thus, a reliance on pure diffusion for conveying protonsfrom the fuel stream to the oxidant stream can be avoided and currentdensities of at least 0.1 mA/cm² can be achieved.

Preferably, an electrochemical cell embodying features of the presentinvention produces current densities of at least 0.1 mA/cm², morepreferably of at least 1 mA/cm², still more preferably of at least 2mA/cm². A current density of 27 mA/cm² has been produced in accordancewith presently preferred embodiments. Although there is presently nopreferred limit to the amount of current density produced by anelectrochemical cell embodying features of the present invention, it ispreferred that the current density produced by a cell be substantiallymatched to the requirements for a particular application. For example,if an electrochemical cell embodying features of the present inventionis to be utilized in a cellular phone requiring a current density ofabout 10 mA/cm², it is preferred that the electrochemical cell produce acurrent density that is at least sufficient to match this demand.

Advantages of the design shown in FIG. 7 include but are not limited tothe following: reduced cost due to the elimination of a PEM; increasedcell lifetime due to the continual regeneration of the IDCI, whichneither wears out nor fails under flow; reduced internal resistance ofthe cell due to the infinite thinness of the IDCI; reduction orelimination of methanol crossover or fouling of the cathode since, withproper design, diffusion occurs only downstream of the cathode; abilityto recycle back into the fuel stream left-over methanol that crossesover into the oxidant stream; ability to increase reaction kineticsproportionally with temperature and/or pressure without compromising theintegrity of the IDCI; ability to fabricate a highly efficient,inexpensive, and lightweight cell through optimization of celldimensions, flow rate, fuel (concentration and composition), oxidant(concentration and composition) and electrodes (surface area, activity,and chemical composition).

In an optimized cell design, the methanol is completely consumed beforeit diffuses into the oxidant stream. This is feasible if theconcentration of methanol is controlled by a methanol sensor coupled toa fuel injector or to a flow rate monitor. Alternatively, a waterimmiscible oxidant fluid stream having a very low affinity for methanoland a high affinity for oxygen and carbon dioxide can be used inconjunction with the laminar flow-type cell shown in FIG. 7. At leastone such family of fluids (viz., perfluorinated fluids such asperfluorodecalin available from F2 Chemicals Ltd., Preston, UK) has beensuccessfully used in respiration-type fluids for medicinal applications.These fluids exhibit an extremely high affinity for oxygen and extremelylow affinities for methanol and water. They are chemically inert andthermally stable. When these fluids are doped with NAFION or analternative proton source, they become proton conducting. Thus, inasmuchas methanol is soluble in the aqueous fuel stream only, the unwantedproblem of methanol crossover into the water immiscible oxidant fluidstream is reduced or eliminated. Moreover, since both liquids areexcellent heat exchangers, an external cooling system is not required.

Cell and electrode dimensions and electrode placement affect cellefficiency. FIG. 8 shows two alternative cell designs. In FIG. 8A, theanode and cathode are positioned side-by-side, analogous to theplacement shown in FIG. 7. In FIG. 8B, the anode and cathode arepositioned face-to-face. The optimization of cell dimensions can beachieved via computer modeling (e.g., using fluid flow modelingprograms, Microsoft EXCEL software, etc.) to correlate optimum laminarflow conditions (i.e., minimum mixing) with easily fabricated channeldimensions and geometries. Critical values for the Reynolds number canbe calculated for an array of cell designs with respect to channelwidth, depth, length, flow rate, and interfacial surface area. In thismanner, a channel design that provides the greatest power output andhighest fuel conversion can be determined.

When appropriate electrode dimensions and placement of electrodes havebeen determined as set forth above, the electrodes are then patternedonto a support (e.g., a soda lime or pyrex glass slide). The electrodesmay be sacrificial electrodes (i.e., consumed during the operation ofthe electrochemical cell) or non-sacrificial electrodes (i.e., notconsumed by the operation of the electrochemical cell). In preferredembodiments, the electrodes are non-sacrificial. In any event, the typeof electrode used in accordance with the present invention is notlimited. Any conductor with bound catalysts that either oxidize orreduce methanol or oxygen are preferred. Suitable electrodes include butare not limited to carbon electrodes, platinum electrodes, palladiumelectrodes, gold electrodes, conducting polymers, metals, ceramics, andthe like.

The electrode patterns can be produced by spray coating a glass slideand mask combination with dispersions of metallic (preferably platinum)particles in an organic or aqueous carrier. A preferred dispersion ofplatinum particles in an organic carrier is the inexpensive paintproduct sold under the tradename LIQUID BRIGHT PLATINUM by WaleApparatus (Hellertown, Pa.). The patterned slide is then baked in a hightemperature oven in the presence of oxygen or air to produce a thinconductive layer of pure platinum. This technique enables production ofthin, high surface area, mechanically robust, low resistance, platinumelectrodes on glass slides. To increase the carbon monoxide tolerance ofthese electrodes, they can be decorated with ruthenium using chemicalvapor deposition, sputtering, or a technique known as spontaneouselectroless deposition (see: A. Wieckowski et al. J. Catalysis, 2001, inpress).

Once the electrodes have been patterned on a support, the microchannelcan be constructed readily from flat, inexpensive, precision startingmaterials as shown in FIGS. 9-10 using techniques such as thosedescribed by B. Zhao, J. S. Moore, and D. J. Beebe in Science, 2001,291, 1023-1026. Microchannel 34 can be constructed from commerciallyavailable glass slides 36 and cover slips 38. The microchannel 34 can besealed with an ultraviolet-based chemically resistant adhesive. Apreferred ultraviolet-based chemically resistant adhesive is that soldby Norland Products, Inc. (Cranberry, N.J.), which is chemicallyresistant to most water-miscible solvents. The cell thus produced willhave chemical resistance and can be employed as a single channel laminarflow DMFC.

Once a single channel laminar flow DMFC has been assembled, optimizationexperiments can be performed in which the efficiency of the cell isevaluated with respect to concentration of methanol, concentration ofproton, oxidant composition, flow rate, and temperature. Evaluation ofcell performance is determined based on cell potential, current density,peak power, and power output. The single channel laminar flow DMFC isreusable, and multiple experiments can be performed with the same cell.

The fuel and oxidant are introduced into the flow channel with the aidof one or more pumps, preferably with the aid of one or morehigh-pressure liquid chromatography (HPLC) fluid pumps. For example, theflow rate of the fuel and oxidant streams can be controlled with twoHPLC pumps to enable precise variation of the flow rate from 0.01 to 10mL/min. This approach allows for the use of large reservoirs of fuel andoxidant that can be heated to constant temperatures and maintained underinert atmosphere, air, or oxygen, as needed. The effluent streams can bemonitored for the presence of methanol to quantify chemical conversion,cell efficiency, and methanol crossover, by sampling the effluent streamand subjecting it to gas chromatographic analysis. In this manner, theoptimized operating conditions for a single channel laminar flow DMFCcan be determined.

It is noted that the fabrication technique described above can bereadily extended to the construction of multi-channel laminar flow DMFCstacks for use in devices having increased power requirements. Likewise,the methods described above for optimizing and quantifying theefficiency of single channel laminar flow DMFCs can be used to optimizeand quantify the efficiency of arrayed multi-channel cell designs. Theelectrodes in such multi-channel cell designs can be connected in bothseries and parallel configurations to investigate the parameters ofmaximum cell voltage and current.

A single channel laminar flow DMFC can be constructed using materialswith sufficient structural integrity to withstand high temperaturesand/or pressures. Graphite composite materials (similar to those used inDMFCs from Manhattan Scientific) or ceramic materials (similar to thoseused in DMFCs from Los Alamos National Laboratory) can be used in viewof their light weight, mechanical integrity, high temperature stability,corrosion resistance, and low cost. In addition, a variety offabrication techniques can be used to produce the microchannel includingmicro-milling, micro-molding, and utilizing an Electric DischargeMachine (EDM) such as is used in the fabrication of injection molds. Theelectrodes can be deposited as described above, and a chemically inertgasket used to seal the cell. The gasket can be made, for example, froma fluoropolymer such as polytetrafluoroethylene sold under the tradenameTEFLON by DuPont (Wilmington, Del.). Alternative sealing techniques suchas those utilized by Manhattan Scientifics can also be employed.Optimization and quantification of the efficiency of these singlechannel laminar flow DMFCs can be achieved using the techniquesdescribed above.

Although the manner of establishing and utilizing an induced dynamicconducting interface in accordance with the present invention has beendescribed primarily in reference to a DMFC, it is emphatically notedthat the concepts and principles described herein are general to allmanner of electrochemical cells, including but not limited to othertypes of fuel cells and to batteries, photocells, and the like.

The manner in which a device embodying features of the present inventionis made, and the process by which such a device is used, will beabundantly clear to one of ordinary skill in the art based upon jointconsideration of both the preceding description, and the followingrepresentative procedures. It is to be understood that many variationsin the presently preferred embodiments illustrated herein will beobvious to one of ordinary skill in the art, and remain within the scopeof the appended claims and their equivalents.

EXAMPLES

A Laminar Flow Cell Using Sacrificial Electrodes

Flat copper and zinc electrodes (ca. 0.125×20×3 mm) were imbedded into ablock of polycarbonate by micro-machining channels and adhering theelectrodes into these channels to create a flat surface. The electrodeswere both of equivalent size and ran parallel to each other with a gapof approximately 5 mm therebetween. On top of this electrode assemblywas assembled a flow channel composed of microscope coverglass as shownin FIG. 11. The cell was sealed with UV glue (Norland Products Inc.,Cranberry, N.J.) and the input adapters were secured with commerciallyavailable epoxy (Loctite Quick Set Epoxy, Rocky Hill, Conn.). Once thecell was assembled, aqueous solutions of 2M copper sulphate and zincsulphate were prepared. The zinc sulphate solution was brought into thechannel first over the zinc electrode with the aid of a syringe pump(this filled the entire channel with liquid and care was take to removeall air bubbles). The copper sulphate solution was then introduced overthe copper electrode. Laminar flow was established between theelectrodes and a current to voltage plot was developed as shown in FIG.11. The flow rates of the two solutions were held constant and equal toeach other (e.g., at 0.1 mL/min) in order for the induced dynamicconducting interface to exist between the two electrodes. If the flowrates were different and the opposing stream touched the oppositeelectrode, the cell would short and produce no current. Thus, inaccordance with the present invention, it is preferred that the flowrates of the two solutions be similar (i.e., differ by less than about15 percent, more preferably by less than about 10 percent, and stillmore preferably by less than about 5 percent).

A Laminar Flow Cell Using Non-Sacrificial Electrodes

Two flat platinum electrodes (ca. 0.125×20×3 mm) were imbedded into ablock of polycarbonate by micro-machining channels and adhering theelectrodes into these channels, creating a flat substrate with exposedelectrode surfaces. The electrodes were both of equivalent size and ranparallel to each other with a gap of approximately 5 mm. On top of thiselectrode assembly was assembled a flow channel composed of double sticktape and a microscope coverglass as shown in FIG. 11. The cell wassealed and the input adapters were secured with commercially availableepoxy (Loctite Quick Set Epoxy, Rocky Hill, Conn.). Next, solutions ofiron (II) chloride in 10% H₂SO₄ (0.6M) and potassium permanganate in 10%H₂SO₄ (0.076M) were prepared. The iron solution was brought into thechannel first over the platinum electrodes with the aid of a syringepump (this filled the entire channel with liquid and care was take toremove all air bubbles). The permanganate solution was then introducedand laminar flow was visibly established between the electrodes. Theflow rates of the two solutions were held constant and equal to eachother in order for the induced dynamic conducting interface to existbetween the two electrodes. Current flow (I) and cell potential (V) weremeasured with the aid of a variable resistor and potentiometer. Acurrent to voltage plot was then developed as shown in FIG. 12, thusconfirming the functioning of the device as an electrochemical cell. Theflow rate was held at 0.05 mL/min and the reproducibility was good. Thepower plot for this data can also be seen in FIG. 12. Theelectrochemical half reactions for the cell are as follows:$\frac{\begin{matrix}{\left. {{{MnO}_{4}^{-}({aq})} + {8{H^{+}({aq})}} + {5e^{-}}}\rightarrow{{{Mn}^{2 +}({aq})} + {4H_{2}O\quad E^{{^\circ}}}} \right. = {1.507V}} \\{\quad {\left. {5\quad {{Fe}^{2 +}({aq})}}\rightarrow{{5\quad {{Fe}^{3 +}({aq})}} + {5\quad e^{-}\quad E^{{^\circ}}}} \right. = {{- 0.75}\quad V}}}\end{matrix}}{\quad {E_{cell}^{{^\circ}} = {0.75\quad V}}}$

This particular chemistry was chosen to demonstrate the feasibility of areaction in which all products and reactants remained in solution andutilized a common electrolyte. Since the electrodes are not involved inthe reaction, their lifetimes are very long and the cell will continueto operate as long as oxidant and reductant are provided. The IDCI hasan infinite lifetime because it is constantly being regenerated underflow. With this particular reaction, the dark purple permanganatesolution becomes colorless at the cathode under high current conditionsproviding a visible means of measuring current flow. Should the effluentstream be purple, it indicates that oxidant has not been completelyconsumed. The color change occurs only at the cathode surface (not atthe interface), further indicating true laminar flow with ionconductivity. This technology can be transferred directly toapplications involving DMFCs.

The laminar flow induced dynamic conducting interface technologydescribed herein is applicable to numerous cells systems including butnot limited to batteries, fuel cells, and photoelectric cells. It iscontemplated that this technology will be especially useful in portableand mobile fuel cell systems, such as in cellular phones, laptopcomputers, DVD players, televisions, palm pilots, calculators, pagers,hand-held video games, remote controls, tape cassettes, CD players, AMand FM radios, audio recorders, video recorders, cameras, digitalcameras, navigation systems, wristwatches, and the like. It is alsocontemplated that this technology will also be useful in automotive andaviation systems, including systems used in aerospace vehicles and thelike.

Throughout this description and in the appended claims, it is to beunderstood that elements referred to in the singular (e.g., amicrochannel, a fuel cell, a spacer, a fuel input, an oxidant input, andthe like), refer to one or a plurality of such elements, regardless ofthe tense employed.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. An electrochemical cell comprising: a firstelectrode a second electrode; and a channel contiguous with at least aportion of the first and the second electrodes; such that when a firstliquid is contacted with the first electrode, a second liquid iscontacted with the second electrode, and the first and the secondliquids flow through the channel, a parallel laminar flow is establishedbetween the first and the second liquids, and a current density of atleast 0.1 mA/cm²is produced.
 2. The electrochemical cell of claim 1wherein the channel comprises a first input adjacent to the firstelectrode, and a second input adjacent to the second electrode.
 3. Theelectrochemical cell of claim 2 wherein the first liquid is introducedthrough the first input, and the second liquid is introduced through thesecond input.
 4. The electrochemical cell of claim 3 wherein the firstliquid is introduced through the first input using a first pump, and thesecond liquid is introduced through the second input using a secondpump.
 5. The electrochemical cell of claim 3 wherein the first liquid isintroduced through the first input at a first flow rate, the secondliquid is introduced through the second input at a second flow rate, andthe first flow rate and the second flow rate are similar.
 6. Theelectrochemical cell of claim 2 wherein the channel further comprises afirst outlet adjacent to the first electrode and a second outletadjacent to the second electrode.
 7. The electrochemical cell of claim 2wherein the first input and the second input are configured such that afirst liquid introduced through the first input and a second liquidintroduced through the second input will be in physical contact along aninterface in parallel laminar flow.
 8. The electrochemical cell of claim7 wherein the support comprises polycarbonate.
 9. The electrochemicalcell of claim 7 wherein the support comprises a glass slide.
 10. Theelectrochemical cell of claim 1 wherein the first liquid comprisesmethanol and the second liquid comprises oxygen.
 11. The electrochemicalcell of claim 1 wherein a proton gradient is established along at leasta portion of an interface between the first liquid and the secondliquid.
 12. The electrochemical cell of claim 11 wherein a diffusion ofprotons from the first electrode to the second electrode occursprimarily downstream of the second electrode.
 13. The electrochemicalcell of claim 1 wherein the first liquid comprises methanol and thesecond liquid comprises hydrogen peroxide.
 14. The electrochemical cellof claim 1 wherein the channel has a substantially straight flow channelgeometry.
 15. The electrochemical cell of claim 1 further comprising asupport having a surface with first and second recessed portions,wherein the first and the second electrodes occupy the first and secondrecessed portions, respectively, such that an upper surface of the firstelectrode and an upper surface of the second electrode are planar withthe surface of the support.
 16. The electrochemical cell of claim 1wherein the first liquid and the second liquid are miscible.
 17. Theelectrochemical cell of claim 1 wherein the first liquid and the secondliquid are immiscible.
 18. The electrochemical cell of claim 1 whereinthe first electrode and the second electrode are spray-coated on asupport.
 19. The electrochemical cell of claim 1 wherein each of thefirst electrode and the second electrode comprises platinum.
 20. Theelectrochemical cell of claim 19 wherein at least one of the firstelectrode and the second electrode comprises ruthenium.
 21. Theelectrochemical cell of claim 1 wherein the first and the secondelectrodes are electrically coupled.
 22. The electrochemical cell ofclaim 1 wherein the first liquid comprises a fuel and the second liquidcomprises an oxidant.
 23. The electrochemical cell of claim 1 whereinthe first liquid comprises a fuel whose concentration is controlled by afuel sensor coupled to a device selected from the group consisting of afuel injector, a flow rate monitor, a fuel recycler, a gas exchanger,and combinations thereof.
 24. The electrochemical cell of claim 23wherein the fuel comprises methanol and the fuel sensor comprises amethanol sensor.
 25. The electrochemical cell of claim 1 wherein thesecond liquid comprises an oxidant and is mechanically coupled to adevice selected from the group consisting of a gas exchanger, an oxidantinjector, an oxidant reservoir, and combinations thereof.
 26. Theelectrochemical cell of claim 1 wherein the first electrode comprises ananode and the second electrode comprises a cathode.
 27. Theelectrochemical cell of claim 1 wherein the cell constitutes a fuelcell.
 28. The electrochemical cell of claim 27 wherein the fuel cellcomprises a direct methanol fuel cell.
 29. A device comprising theelectrochemical cell of claim
 1. 30. A portable electronic devicecomprising the electrochemical cell of claim
 1. 31. A method ofgenerating an electric current comprising operating the electrochemicalcell of claim
 1. 32. A method of generating water comprising operatingthe electrochemical cell of claim
 1. 33. A method of generatingelectricity comprising: flowing a first liquid and a second liquidthrough a channel in parallel laminar flow, wherein the first liquid isin contact with a first electrode and the second liquid is in contactwith a second electrode, wherein complementary half cell reactions takeplace at the first and the second electrodes, respectively, and whereina current density of at least 0.1 mA/cm² is produced.
 34. A fuel cellcomprising a first electrode and a second electrode, wherein ions travelfrom the first electrode to the second electrode without traversing amembrane, and wherein a current density of at least 0.1 mA/cm² isproduced.
 35. In a fuel cell comprising a first liquid containing a fuelin contact with a first electrode, a second liquid containing an oxidantin contact with a second electrode, and a membrane separating the firstand the second electrodes, the improvement comprising replacing themembrane with a parallel laminar flow of the first and the secondliquids, and providing each of the first liquid and the second liquidwith a common electrolyte.
 36. An electrochemical cell comprising: asupport having a surface; a first electrode connected to the surface ofthe support; a second electrode connected to the surface of the supportand electrically coupled to the first electrode; a spacer connected tothe surface of the support, which spacer forms a partial enclosurearound at least a portion of the first and the second electrodes; and amicrochannel contiguous with at least a portion of the first and thesecond electrodes, the microchannel being defined by the surface of thesupport and the spacer; such that when a first liquid is contacted withthe first electrode, and a second liquid is contacted with the secondelectrode, a parallel laminar flow is established between the first andthe second liquids, and a current density of at least 0.1 mA/cm² isproduced.